Single-crystal piezoelectric materials are widely used in industry, notably for producing surface acoustic wave (SAW) or bulk acoustic wave (BAW) components allowing resonators or filters to be synthesized. For these applications the most commonly used piezoelectric substrates are quartz, lithium niobate, lithium tantalate (LiNbO3, LiTaO3) or even langasite substrates. Among these materials lithium niobate and tantalate in addition exhibit relatively large variations in optical index as a function of mechanical stresses, thereby making them candidates of choice for the production of optoacoustic modulators for example.
Because of the bulk nature of the aforementioned substrates, the propagation medium of the acoustic waves can consist only of said substrate and one or more additional, optionally deposited, layers. This therefore only allows surface waves (waves naturally guided at the surface of the substrate), waves guided in a deposited layer, or a wave guided at the interface between the substrate and a deposited layer, to be exploited. In all these cases, the components produced use electrodes, in the form of interdigitated combs deposited on the surface of the substrate, to generate or receive acoustic waves, or even both simultaneously, as shown in FIGS. 1a and 1b, which illustrate a configuration in which the waves are created by interdigitated electrode combs PEi produced directly on the surface of a piezoelectric substrate Spiezo or at the interface between a layer CS and a piezoelectric substrate Spiezo, respectively.
The advantage of surface wave components such as those shown in FIGS. 1a and 1b is that the surface waves are essentially localized in the substrate, thereby making it possible to take advantage of the properties of single-crystal materials: excellent reproducibility of the elastic, dielectric and piezoelectric properties compared to a deposited material; minimization of acoustic losses by virtue of the absence of inhomogeneities (grain boundaries, dislocations, etc.) in the material.
The drawback, in contrast, is that although it is possible to modify the crystal orientation, the propagation speed of the surface waves is set by the constituent material of the substrate, and generally remains relatively low (in the region of 4000 to 5000 m/s). For high-frequency applications (a few gigahertz), the dimensions of the features of the electrodes become extremely small, being related to the wavelength of the mode that it is desired to excite, and therefore become technologically difficult to control. The same problem is encountered in the case of interface waves.
One solution to this decrease in the dimensions of the interdigitated combs consists in using waves having a higher propagation speed than that of surface waves. This is the case of waves guided in a layer deposited on the surface of the substrate. Such waves can in theory reach virtually infinite propagation speeds. More reasonably, it is possible to exploit waves having phase speeds of about a few tens of kilometers per second, thereby already enabling, at equal frequency, constraints on the definition of the features of the electrodes to be considerably relaxed.
The major drawback in this case is that these waves are essentially localized in the added layer. This has two consequences: firstly, the wave propagates in a medium that no longer is single-crystal in nature, and therefore exhibits higher losses than a surface wave, even if perfect confinement of these waves is assumed. Secondly, the transduction (conversion of electrical energy into mechanical energy and vice versa) occurs in the piezoelectric material, even though most of the acoustic energy is localized in the deposited layer. Therefore, as a result the efficiency of this transduction is considerably reduced, thereby decreasing the performance of the final component (bandpass filters with narrower bands, resonators exhibiting less marked resonances, etc.).
It is also possible to exploit what are called bulk waves, which propagate in the thickness direction of the substrate. These waves are excited by electrodes—one electrode called the upper electrode ES and one electrode called the lower electrode Ei—surrounding the piezoelectric substrate Spiezo, as shown for example in FIG. 2a. The resonant frequency is in this case directly related to the thickness of the piezoelectric wafer. For low-frequency applications (a few megahertz), the substrate must typically be about 300 to 500 μm in thickness.
At higher frequencies, it becomes necessary to use smaller thicknesses, as small as a few hundreds of nanometers for applications at frequencies of about a gigahertz. This may be obtained by local etching of the substrate so as to form a membrane with the desired thickness as illustrated in FIG. 2b, which shows the substrate thinned to form a piezoelectric layer Cpiezo, the waves being excited by an upper electrode Es and a lower electrode Ei. This technique is currently employed for quartz resonators, but it is difficult to apply to materials such as LiTaO3 or LiNbO3 because of their very high resistance to the various chemical or physical etching techniques available.
In addition, because of the precision with which the thicknesses must be obtained, approaches using local thinning have historically been abandoned. Currently it is preferred to employ thin piezoelectric film deposition techniques, which techniques provide much better uniformity and control of the thickness. Components produced in this way are called FBARs (film bulk acoustic resonators) when a suspended piezoelectric membrane is formed, or SMRs (solidly mounted resonators) when the acoustic confinement is obtained using a mirror composed of multiple stacked layers (analogous to optical Bragg mirrors). Even though these components have seen considerable growth in the last 10 years they still have a number of major drawbacks:                piezoelectric-film deposition techniques do not allow the crystal orientation of the material to be easily chosen. In practice only a single orientation is therefore possible, thereby imposing precise characteristics (polarization, propagation speed, etc.) on the waves exploited;        the properties of the piezoelectric materials conventionally employed for this type of component, namely aluminum nitride (AlN) or zinc oxide (ZnO), are greatly inferior to those of the materials employed in surface wave applications (LiNbO3 or LiTaO3), thereby preventing synthesis of very wideband filters, resonators with very high surtension coefficients, etc.        
More recently, thin-film transfer techniques, such as for example combinations of bonding/thinning or even the SmartCut process concerning LiNbO3, as described in the article M. Pijolat, A. Reinhardt, E. Defay, C. Deguet, D. Mercier, M. Aïd, J-S Moulet, B. Ghyselen, D. Gachon, S. Ballandras “Large Q.f product for HBAR using SmartCut transfer of LiNbO3 thin layers onto LiNbO3 substrate”—Proceedings of the 2008 IEEE Ultrasonics Symposium, p 201-204, have allowed thin single-crystal piezoelectric films (a few hundred nanometers in thickness) to be formed joined to other substrates, while preserving the intrinsic properties of the original material. Some of the aforementioned drawbacks can therefore be solved by these techniques. Nevertheless, the processes used impose technological constraints that at the present time remain insurmountable:                the “host” substrate (the substrate of the final component) and the “donor” substrate (that from which the added layer is produced) must be able to be bonded to one another. They must therefore both be flat. They must also have surfaces that are compatible with the bonding; and        because of differences in the expansion coefficients of the added layer and the host substrate, it is not always possible to use any type of bonding, especially if it is desired not to use an elastic layer to absorb differential expansions. Thus, it is difficult to add layers of LiNbO3 to a silicon substrate and it is preferable to add a layer of LiNbO3 to a substrate made of the same material in order to overcome these effects. This constraint greatly reduces the number of possible stacks possible, and at the present time is slowing development of SMR devices using added layers.        